Influence of crystallization kinetics on the cellular structure of biodegradable PLA foams during supercritical foaming

Materials and characterization techniques

Poly (lactic acid) (PLA) was selected as the primary biodegradable polymer in this study due to its commercial accessibility and clear crystallization behavior. The material was processed into pelletized form and dried prior to foaming to minimize moisture-induced degradation. Melt flow characteristics were evaluated using a capillary rheometer to determine flow stability and viscosity under foaming-relevant thermal conditions, which are essential for maintaining structural integrity during cell growth. To investigate crystallization kinetics and structural evolution, a combination of thermal and structural characterization techniques was employed. Differential scanning calorimetry (DSC) was utilized to analyze non-isothermal and isothermal crystallization behavior, enabling calculation of crystallization parameters based on Avrami and Lauritzen–Hoffman models. Polarized optical microscopy (POM) provided insights into spherulite nucleation density and growth dynamics under controlled heating and cooling rates. Small- and wide-angle X-ray scattering (SAXS/WAXS) was applied in in-situ mode to monitor lamellar thickening, long period evolution, and crystal orientation during gas penetration and crystallization. Additionally, Fourier-transform infrared spectroscopy (FTIR) was used to examine potential CO₂-induced chemical interactions, while gel permeation chromatography (GPC) confirmed molecular weight stability after processing. Density and porosity changes before and after foaming were recorded using the Archimedes method for quantitative correlation with cellular structure characteristics.

Supercritical CO₂ foaming procedure

Supercritical CO₂ foaming experiments were conducted in a high-pressure batch reactor with precise control over saturation pressure and temperature. Commercial PLA pellets were first dried in a vacuum oven at 60°C for 12 h to remove residual moisture. Dried PLA pellets (10.0 g for each batch) were compression-molded into rectangular specimens with dimensions of 20 × 10 × 2 mm³ at 180°C under 10 MPa for 5 min, followed by cooling to room temperature under pressure. Before foaming, the molded PLA specimens were cleaned with ethanol, dried under vacuum at 40°C for 6 h, and stored in a desiccator to avoid moisture absorption.

For each foaming experiment, approximately 2.0 g of PLA specimens were placed in the reactor. Liquid CO₂ was introduced using a high-pressure pump until the preset saturation pressure was reached. The samples were saturated at selected temperatures and pressures to obtain target CO₂ uptake levels of 0 wt%, 5 wt%, and 10 wt%. After saturation equilibrium was reached, rapid depressurization was performed by opening the pressure-release valve within 3–5 s to induce cell nucleation and growth. The foamed samples were immediately cooled to room temperature to stabilize the cellular morphology. The residual CO₂ was allowed to desorb naturally under ambient conditions for 24 h before structural characterization.

Accordingly, these three CO₂ contents were designed to represent three characteristic states of PLA during foaming: non-plasticized PLA, moderately plasticized PLA, and highly plasticized PLA. This design enables systematic evaluation of how CO₂ dissolution affects crystallization kinetics, gas-cell nucleation, and final foam morphology. CO₂ content was determined using the mass difference method:

C C O 2 = m s − m 0 m 0 × 100

(1)

Rapid pressure release triggered phase separation, enabling bubble formation throughout the matrix, while subsequent cooling stabilized the cellular architecture by preventing post-growth coalescence and gas escape. During this process, crystallinity and crystallization kinetics were purposely regulated through thermal pretreatments and controlled cooling rates, creating different solid-phase constraints during foaming. Key variables included initial crystallinity, crystallization rate, foaming temperature relative to the glass transition region, and gas dissolution content, all of which influence nucleation density and cell expansion efficiency. After foaming, samples were stored under ambient conditions for structural relaxation prior to characterization.

Scanning electron microscopy (SEM) was used to evaluate cell morphology after cryogenic fracture, including average cell diameter, cell density, and wall thickness. Structural parameters were quantified through image analysis software to ensure statistical reliability. The combined experimental approach enabled the establishment of a direct relationship between crystallization dynamics and the formation of the final porous structure in PLA foams processed by supercritical CO₂.

Microstructural characterization and crystallization kinetic modeling

The cellular morphology of the foamed PLA samples was examined through scanning electron microscopy (SEM). Samples were cryo-fractured in liquid nitrogen to preserve internal features and sputter-coated with a thin gold layer to improve conductivity. SEM images were analyzed using ImageJ software to quantify average cell diameter, size distribution width, and structural uniformity across different treatment conditions. The open-cell ratio was estimated by comparing fractured surface porosity with bulk density differences, enabling assessment of gas escape pathways during expansion. Foam density (ρf) was determined using the Archimedes displacement method. Based on measured density values, the cell number density (Nc) was calculated using the established relationship:

N c = ( n A ) 3 2 ( ρ s ρ f )

(2)

To evaluate crystallization behavior quantitatively, DSC data were fitted using the Avrami kinetic model, which describes the time-dependent variation of relative crystallinity:

X ( t ) = 1 − exp ( − k t n )

(3)

The density of solid PLA ( ρ s ) and the apparent density of foamed PLA samples ( ρ f ) were measured to support quantitative analysis of cellular morphology. For each specimen, the dry mass in air ( m a ) was first measured using an analytical balance with a resolution of 0.1 mg. The sample was then slowly immersed into the liquid medium and gently agitated to remove trapped surface bubbles. To avoid water penetration into open pores during prolonged immersion, the immersed mass ( m i ) was recorded after a short equilibrium period of approximately 60 s, once the balance reading became stable. For highly porous foams, the sample was held using a thin stainless-steel suspension wire, and the buoyancy contribution of the wire was measured separately and subtracted from the final value. Each measurement was repeated at least three times using independent specimens.

The sample density ( ρ ) was calculated according to:

ρ = m a m a − m i ρ l

(4)

P = ( 1 − ρ f ρ s ) × 100 %

(5)

N c = ( n A ) 3 / 2 ( ρ s ρ f )

(6)

Crystallization activation energy calculation

The apparent activation energy of non-isothermal crystallization was calculated from DSC cooling scans performed at different cooling rates. The DSC measurements were conducted using a TA Instruments Q2000 differential scanning calorimeter (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere with a flow rate of 50 mL min^-1. Before testing, the instrument was calibrated using high-purity indium and zinc standards to ensure temperature and enthalpy accuracy. The temperature fluctuation during the test was controlled within ±0.1°C to minimize thermal measurement errors.

For each measurement, PLA samples of approximately 5–8 mg were sealed in aluminum pans. The samples were heated to 190°C at 10°C min^-1 and held for 3 min to eliminate previous thermal history, followed by cooling to 30°C at controlled cooling rates of 5, 10, 15, and 20°C min^-1. The crystallization peak temperature ( T p ) was recorded from each DSC cooling curve. Each condition was tested at least three times, and the average value was used for activation energy calculation.

The Kissinger method was used as the primary approach to determine the crystallization activation energy ( E a ) , according to:

ln ( β T p 2 ) = − E a R T p + C

(7)

To further validate the activation energy trend, the Flynn–Wall–Ozawa (FWO) method was also applied:

log β = − 0.4567 E a R T p + C ′

(8)

Data analysis strategy

The analytical strategy focused on establishing a mechanistic mapping between crystallization behavior and foam microstructure. Structural metrics such as cell density and wall thickness were evaluated in parallel with crystallization parameters, enabling visualization of how crystal growth influences nucleation density and cell development. Data were grouped based on crystallinity levels to identify threshold values beyond which crystal-induced structural constraints dominate bubble expansion. Emphasis was placed on the role of inter-spherulitic boundaries, where enhanced nucleation tends to occur due to local free-volume concentration, while high crystallinity regions impose a rigid confinement that restricts cell growth and increases structural uniformity.

Special attention was given to the competitive interactions between bubble nucleation sites and spherulite interfaces during the foaming stage. Regions experiencing rapid crystallization often exhibit delayed gas diffusion and reduced cell expansion due to reduced chain mobility. Conversely, moderate crystallization rates promote heterogeneous nucleation and improve the fineness of cell dispersion. By pairing SEM results with kinetic parameters extracted from Avrami analysis, a structure-to-process mapping was developed that distinguishes the relative contribution of nucleation acceleration and growth suppression. This integrative evaluation supports the identification of controllable crystallization regimes capable of tailoring foam morphology for targeted performance, such as enhanced mechanical stability or increased gas retention efficiency.

Mechanistic interpretation of crystallization kinetics under CO₂-Assisted conditions

The preliminary characterization results in Figure 1 provide the structural and thermal basis for analyzing crystallization-regulated PLA foaming behavior. The DSC thermogram in Figure 1(a) shows a typical thermal response of semi-crystalline PLA, with a glass transition at approximately 60.8°C, a distinct cold crystallization peak at 112.4°C, and a melting endotherm at 167.3°C. The presence of a pronounced cold crystallization peak indicates that the PLA matrix still contains a considerable amorphous fraction after thermal history treatment, suggesting that molecular chains retain sufficient mobility to rearrange during subsequent CO₂-assisted foaming. The calculated crystallinity of 28.6% further confirms that the original PLA possesses an intermediate crystallization state, which is favorable for investigating the competition between crystal growth and gas-cell expansion.OPEN IN VIEWER

The polarized optical microscopy image in Figure 1(b) reveals densely packed spherulitic structures with clear birefringence, indicating that PLA develops well-defined crystalline domains under controlled crystallization conditions. The average spherulite diameter of approximately 18 μm suggests a relatively refined crystalline morphology rather than large isolated spherulites. Such a crystalline texture is important for foam formation because spherulite boundaries can act as heterogeneous nucleation sites during rapid depressurization. These interfacial regions may locally concentrate free volume and dissolved CO₂, thereby lowering the energy barrier for bubble nucleation and contributing to a more uniform cellular structure.

The SEM image in Figure 1(c) shows that PLA foam prepared by supercritical CO₂ foaming exhibits a predominantly closed-cell morphology. The cell walls are relatively continuous, and the pores are distributed without obvious large-scale collapse or severe coalescence. The measured average cell size of 105 ± 18 μm indicates that the foam structure is moderately uniform, which is consistent with the refined spherulitic morphology observed in Figure 1(b). This result suggests that the crystalline phase does not merely act as a passive solid fraction, but provides mechanical support during bubble growth and helps stabilize the pore walls before complete gas escape or structural relaxation occurs.

The cell size distribution in Figure 1(d) further confirms the uniformity of the cellular architecture. Most pores are concentrated between approximately 80 and 130 μm, with a mean value of 105 μm and a calculated cell density of 1.8 × 10 7 cells/cm³. The relatively narrow distribution reflects effective bubble nucleation and restrained cell growth under the selected foaming conditions. Taken together, the DSC, POM, SEM, and statistical morphology results demonstrate that PLA has suitable thermal transition behavior, crystallization capability, and cellular stability for studying crystallization–foaming coupling. These baseline features provide a necessary reference for later analysis of how CO₂-induced plasticization, crystal growth kinetics, and spherulitic interfaces jointly regulate pore refinement and foam performance.

The kinetic parameters extracted from Avrami fitting reveal that CO₂ dissolution introduces a profound modulation of PLA crystallization pathways by simultaneously altering nucleation thermodynamics and chain transport kinetics. As presented in Figure 2(a), the crystallization rate constant ( k ) increases monotonically with temperature, yet its temperature sensitivity intensifies markedly under CO₂-rich conditions. At 140°C, k under 10 wt% CO₂ exceeds that of the neat PLA matrix by more than 59%, evidencing a dual mechanistic contribution: CO₂-enhanced segmental mobility in the amorphous phase accelerates primary nucleation, while its plasticization effect facilitates cooperative chain rearrangement across crystal growth fronts. This synergistic acceleration promotes both the density and continuity of lamellar assemblies. The reduction in induction time observed from DSC thermograms supports this interpretation, indicating that nucleation initiates more homogeneously throughout the bulk rather than being confined to spherulitic interfaces.OPEN IN VIEWER

The variation in Avrami exponent provides additional evidence for the change in crystallization mechanism. As shown in Figure 2(b), n increases from 2.7 to 3.1 with increasing CO₂ content, indicating a transition from sporadic crystallization toward more spatially distributed heterogeneous nucleation. This behavior can be attributed to the plasticization effect of CO₂, which increases free volume and generates more favorable nucleation sites. Consistently, Figure 2(f) shows that the nucleation density nearly doubles as the temperature increases from 100 to 140°C under CO₂-assisted conditions, demonstrating that enhanced molecular mobility contributes directly to the formation of a denser crystalline network.

The reduction in non-isothermal crystallization activation energy from 95 to 82 kJ mol^-1 (Figure 2(c)) further confirms the role of CO₂ in lowering the energetic barrier for molecular ordering. This decrease indicates that PLA chains require less thermal energy to reorganize into crystalline structures when CO₂ is dissolved in the matrix. SAXS results in Figure 2(e) show an increase in the lamellar long period with temperature, suggesting more developed lamellar structures under conditions where chain diffusion is sufficiently activated. These results collectively demonstrate that CO₂ promotes crystallization not only by increasing molecular mobility but also by enabling more effective lamellar organization.

Integrating these structural and kinetic observations leads to a coherent mechanistic model. CO₂-induced plasticization reduces the nucleation barrier and enhances chain diffusivity, while thermal energy promotes lamellar perfection, together yielding a dual-domain microstructure composed of highly nucleated yet mechanically coherent crystalline regions. This hybrid morphology offers both the nucleation density required for bubble initiation and the rigidity needed for structural stabilization during supercritical foaming. When the processing window is tuned such that crystallization proceeds within the optimal kinetic regime—sufficiently rapid to provide reinforcement but not so fast as to restrict gas expansion—the system achieves a self-regulated balance between crystallization and foaming dynamics. This thermodynamic–kinetic synchronization ultimately governs the formation of uniform cell architectures and underpins the superior mechanical and functional performance of CO₂-foamed PLA.

The crystallization activation energy was calculated using the Kissinger method based on DSC cooling scans performed at different cooling rates. The fitting plots of ln ⁡ ( β / T p 2 ) versus 1 / T p showed good linearity, with R 2 values higher than 0.99 for all samples, indicating that the Kissinger model is suitable for describing the non-isothermal crystallization behavior of PLA under the selected conditions. The calculated activation energy decreased from 95.0 ± 2.1 kJ / mol for neat PLA to 88.0 ± 1.8 kJ / mol and 82.0 ± 1.6 kJ / mol for PLA containing 5 wt% and 10 wt% CO₂, respectively. The FWO method produced a consistent activation energy trend, although the absolute values were slightly higher than those obtained from the Kissinger method. This difference is reasonable because the two approaches are derived from different approximations of non-isothermal crystallization kinetics. The agreement between the two fitting methods confirms that CO₂ dissolution reduces the energetic barrier for crystallization rather than producing an artifact from a single fitting model. The reduction in E a reflects enhanced chain mobility and reduced packing resistance caused by CO₂-induced plasticization, allowing PLA segments to rearrange into ordered lamellae more easily during cooling.

Evolution of foam cellular structure governed by crystallinity-induced nucleation dynamics

Quantitative analysis of SEM-derived structural parameters reveals a hierarchical co-regulation mechanism between crystalline phase development and cellular morphology evolution in CO₂-foamed PLA. As shown in Figure 3(a) and (b), both nucleation density and final cell density increase nonlinearly with crystallinity, exhibiting a pronounced acceleration when crystallinity surpasses approximately 30%. This inflection reflects the onset of an interconnected spherulitic boundary network that functions as an energetically favorable heterogeneous nucleation scaffold during depressurization. The area fraction of such boundaries, increasing from 20% to 47% between 10% and 50% crystallinity (Figure 3(f)), provides dual functionality: it supplies additional interfacial sites for CO₂-induced phase separation and simultaneously concentrates localized strain fields that catalyze bubble initiation. The formation of these interfacial microcavities effectively couples the solid-state crystallization front with gas-phase nucleation kinetics, leading to a more spatially homogeneous distribution of nuclei and consequently a finer, more isotropic pore morphology.OPEN IN VIEWER

The decrease in average cell size shown in Figure 3(c) further confirms the role of crystallinity in limiting bubble growth. As crystallinity increases from 10% to 50%, the average cell size decreases from approximately 165–180 μm to below 80 μm, suggesting that bubble expansion becomes increasingly confined by the crystalline framework. At the same time, Figure 3(d) and (e) show an increase in cell wall thickness and a decrease in open-cell ratio, respectively. These trends indicate that crystalline lamellae reinforce the cell walls and suppress cell coalescence or rupture during expansion. Therefore, the crystalline phase contributes simultaneously to pore refinement and structural stabilization.

Integrating these observations suggests a dual-mechanism model in which crystallinity simultaneously governs nucleation thermodynamics and growth kinetics. Increased crystallinity elevates interfacial nucleation potential by expanding spherulitic surface area, while the resulting lamellar network imposes diffusion and mobility constraints that limit gas expansion. The interplay between these counteracting factors—nucleation enhancement versus growth inhibition—defines the morphological stability window of the foam. Optimal cellular architectures emerge within an intermediate crystallization regime, where nucleation density and matrix rigidity reach dynamic equilibrium. Within this range, bubble growth remains isotropic and well-distributed, yielding foams that combine high cell density, uniform pore size, and robust mechanical integrity. This crystallinity–structure coupling mechanism provides a unified framework for understanding how controlled crystallization kinetics dictates the multiscale organization and property evolution of biodegradable foams under supercritical conditions.

The supplemented density data provide a quantitative basis for evaluating the foam expansion and cellular structure. The density of solid PLA was measured as 1.24 g / c m 3 , while the apparent density of PLA foams increased from 0.18 to 0.29 g / c m 3 as crystallinity increased from 10% to 50%. Accordingly, the calculated porosity decreased from 85.5% to 76.6%, indicating that higher crystallinity restricted volumetric expansion during supercritical CO₂ foaming. This trend is consistent with the observed reduction in average cell size and increase in cell wall thickness. A higher crystalline fraction strengthens the polymer matrix and limits excessive gas-driven expansion, resulting in denser foam structures with lower overall void fraction.

The corrected cell number density values further confirm that crystallinity promotes bubble nucleation despite reducing total porosity. As crystallinity increases, the cell number density rises from 8.0 × 10 6 to 2.40 × 10 7 cells / c m 3 . This result suggests that crystalline domains and spherulitic boundaries provide effective heterogeneous nucleation sites during rapid depressurization. Therefore, the foam structure evolves from a low-density, large-cell morphology to a denser and finer cellular architecture. The inclusion of both ρ s and ρ f values strengthens the quantitative interpretation of the crystallization–foaming relationship and improves the reliability of porosity and cell density calculations (Table 1).

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Table 1.

The density, porosity and cell number density of PLA foam samples with different degrees of crystallinity.

Sample number	Crystallinity (%)	The density of solid PLA ρs (g/cm3)	Foam density ρf (g/cm3)	Porosity (%)	Average aperture (μm)	Cell number density (cells/cm3)
PLA-F10	10	1.24	0.18	85.5	180	8.0 × 106
PLA-F20	20	1.24	0.2	83.9	140	1.28 × 107
PLA-F30	30	1.24	0.23	81.5	105	1.84 × 107
PLA-F40	40	1.24	0.26	79	85	2.24 × 107
PLA-F50	50	1.24	0.29	76.6	70	2.40 × 107

Thermodynamic–kinetic coupling mechanism analysis

The progressive reduction in activation energy with increasing CO₂ dissolution (Figure 4(a)) reflects a nontrivial thermodynamic softening of the PLA matrix, where CO₂ molecules act as transient plasticizers that disrupt intermolecular entanglements and facilitate cooperative chain mobility. This melt-plasticization effect reduces the free-energy barrier for segmental rearrangement, enabling faster nucleation and lamellar ordering under lower thermal activation. Combined with the temperature-driven crystallinity enhancement (Figure 4(b)), the polymer evolves into a dual-phase dynamic state characterized by the coexistence of highly mobile amorphous chains and rapidly propagating crystalline fronts. Within this metastable region, crystallization no longer proceeds linearly with temperature but becomes self-accelerating due to the positive feedback between CO₂-induced free-volume expansion and the release of latent crystallization heat. This thermodynamic–kinetic coupling effectively shifts the foaming regime from diffusion-limited to nucleation-controlled behavior during the early stages of bubble formation.OPEN IN VIEWER

Microstructural evidence supports this coupling mechanism. At moderate CO₂ uptake levels (5–10 wt%), the enhanced crystallization kinetics promote the emergence of smaller, more uniformly distributed cells (Figure 4(c) and (d)), which correlate with an increase in local nucleation density and suppression of uncontrolled coalescence. The concurrent reduction in open-cell ratio (Figure 3(e)) is attributed to the crystallization-induced mechanical reinforcement that stabilizes cell walls against tensile rupture during expansion. In this regime, the developing lamellar framework functions as an adaptive scaffold, dissipating internal gas stresses through elastic deformation and delaying the onset of wall collapse. The strengthening of the crystalline phase, evidenced by a significant rise in wall modulus (Figure 4(f)), signifies that the polymer transitions from a gas-dominated foam to a structurally percolated solid network, where crystallinity governs both load-bearing and diffusion resistance.

Once crystallization proceeds beyond the kinetic equilibrium threshold, the system behavior inverts. Excessively rapid crystal growth depletes the amorphous fraction, causing premature matrix solidification before gas pressure can equilibrate. The DSC profiles in Figure 4(g) exhibit shortened crystallization half-times at high CO₂ content, confirming the kinetic overdrive of nucleation. Correspondingly, SAXS results (Figure 4(h)) reveal dense lamellar stacking and restricted long-period development, indicative of diffusion-limited crystal thickening. This over-crystallized regime induces morphological heterogeneity—uneven pore size distribution and localized collapse zones—arising from spatially asynchronous freezing of gas-filled domains. Such kinetic imbalance between solidification and expansion highlights the narrow processing tolerance inherent to CO₂-foamed biodegradable systems.

The 3D schematic in Figure 4(i) conceptually illustrates this coupling behavior: advancing crystalline interfaces form a confining network that entraps bubbles within spherulitic “cages,” where interfacial stress gradients dictate the directionality of further growth. The optimal foaming window therefore exists at the intersection of two counteracting phenomena—CO₂-induced plasticization that enhances crystallization kinetics and lamellar solidification that constrains expansion. Within this equilibrium zone, the structural evolution achieves synchronized crystallization and gas diffusion, yielding a foam microarchitecture that combines isotropic pore dispersion with high mechanical integrity. This thermodynamic–kinetic synergy establishes a predictive framework for designing next-generation biodegradable foams where microstructure and performance emerge from deliberately tuned crystallization dynamics.

Multiscale structure–performance relationships

The evolution of mechanical properties in PLA foams reflects a tightly interwoven relationship between multiscale structural organization and macroscopic deformation behavior. As crystallinity rises from 10% to 50%, a systematic increase in density is observed (Figure 5(a)), corresponding to the progressive transformation of the cellular architecture from a gas-dominated to a crystal-reinforced framework. This densification process signifies not merely a reduction of void fraction but a transition toward a more mechanically percolated morphology in which crystalline lamellae act as rigid junctions within the foam network. The associated refinement in average cell size and increase in lamellar connectivity enhance the local stiffness of the cell walls and improve stress transfer efficiency across adjacent cells. Consequently, the compressive modulus nearly triples within the same crystallinity range (Figure 5(b)), marking the transition from localized bending-dominated deformation to a more collective load-bearing response controlled by the crystalline skeleton.OPEN IN VIEWER

The strengthening mechanism is accompanied by a pronounced enhancement in energy dissipation capacity. As shown in Figure 5(c), the absorbed energy increases from 0.9 MJ m^-3 to 3.0 MJ m^-3 as crystallinity increases, indicating that the crystalline framework not only stiffens the structure but also delays the onset of catastrophic buckling. The stress–strain profiles (Figure 5(e)) reveal a qualitative change in deformation mode: amorphous foams display early collapse and brittle densification, whereas semi-crystalline foams sustain a stable plateau regime over an extended strain range. This mechanical resilience arises from the hierarchical microstructure, where lamellar reinforcement distributes local stress and prevents the coalescence of microcracks. Crystalline domains act as internal “anchors” that store and dissipate energy through elastic–plastic deformation of the surrounding amorphous matrix, transforming the collapse mechanism from abrupt failure to controlled, progressive compression. Morphological uniformity plays a crucial role in sustaining this mechanical balance. The pore-size distribution narrows markedly with increasing crystallinity (Figure 4(d)), reducing local stress gradients and enabling more homogeneous deformation. The negative correlation between average cell size and compressive modulus (Figure 5(f)) confirms that microstructural refinement drives macro-scale strengthening. Smaller, more uniform cells restrict strain localization, resulting in an energetically favorable load distribution that transforms individual cell-wall bending into coordinated network deformation. The 3D schematic (Figure 5(g)) illustrates this multiscale strengthening paradigm: crystalline lamellae percolate through the foam, forming a quasi-continuous load-bearing skeleton interlinked by gas-filled cells, effectively bridging nanoscale crystallinity with macroscale mechanical performance.

Integrating these results establishes a design framework linking molecular crystallization dynamics to bulk energy absorption performance. The optimal structural window is identified within the intermediate crystallinity regime (30–45%), where the competing effects of stiffness enhancement and ductility retention are balanced. In this range, the foam density (0.23–0.26 g cm^-3) and cell size (<100 µm) collectively yield superior mechanical stability and energy dissipation efficiency. This coupling of microstructural regulation and macroscopic functionality underscores a generalizable strategy for tailoring biodegradable foams toward high-performance applications, including lightweight structural cores, shock-mitigating packaging, and bioresorbable scaffolds, where simultaneous stiffness, toughness, and uniformity are required.